In an exhaust system of a vehicle or the like with an internal combustion engine mounted thereon, there are disposed exhaust system parts such as a muffler aimed to reduce exhaust noise, an exhaust purifying device aimed to purify exhaust gas, and the like. In these exhaust system parts, generally employed is an outer tube formed in more than two layers to restrain radiation sound. For example, in Patent document 1 as cited below, there is disclosed a muffler, wherein a sheet material is wrapped around end plates and partition plates to provide an outer tube, and wherein the sheet material and peripheral flanges of the end plates and partition plates are welded by laser at the outside of the outer tube.
Also, in Patent document 2 as cited below, in a muffler for an internal combustion engine with a cylindrical portion of a shell formed in a double cylinder structure of an inner tube and an outer tube, there is proposed the muffler, wherein a vent passage is formed between an outer peripheral surface of the inner tube and an outer peripheral surface of the outer tube, to extend upward along a periphery of the shell from a lower part thereof, and wherein a through hole is formed at a higher position than a predetermined position of the inner tube, to communicate an upper end section of the vent passage with the through hole. As for its background, it is described that if the cylindrical portion of the shell is formed in the double cylinder structure, water may stay in its lowest section between the outer tube and the inner tube, and if the staying water is heated to provide steam, the lower section of the outer tube might be bulged downward by pressure of the steam. Therefore, it is proposed to prevent its deformation by introducing the steam into the muffler through the vent passage and through hole. In this respect, the outer tube as described in the above Patent document 1 corresponds to the shell as described in the above Patent document 2, while it is different from the outer tube constituting a part of the shell. Furthermore, in Patent document 3, there is disclosed a method for manufacturing a muffler, wherein a steel sheet is rolled around an outer periphery of an inner pipe, and processed by a lock-seaming process.
Patent document 1:
Japanese Patent Laid-open Publication No. 59-162318
Patent document 2:
Japanese Patent Laid-open Publication No. 2004-285969
Patent document 3:
Japanese Patent Laid-open Publication No. 6-269884